1. Field of the Invention
The present invention relates to an electrophotographic photoconductor by which image forming is performed by electrostatic copying process of copiers, facsimiles, printers and the like, and image forming apparatus, image forming method and process cartridge using the electrophotographic photoconductor.
2. Description of the Related Art
In recent years, developments of information processing system employing electrophotography are remarkable. Particularly the optical printers by which information is converted to digital signals to be recorded by light have been notably improved in terms of printing quality and reliability. The digital recording technique of this type is also applied to general copiers as well as printers and what is called “digital copiers” have been developed. Moreover, the demand for the copiers, the conventional copiers which have been provided with the digital recording technique is expected to increase more in the future because of additional various information processing functions. Furthermore, developments of digital color printers for performing output of color images and documents are drastically advancing with popularization and upgrade of personal computers.
The electrophotographic photoconductor used in the image forming apparatuses as described above can by classified broadly into organic photoconductor and inorganic photoconductor. Organic photoconductors are being widely used recently because it can be manufactured easily and inexpensively as compared with the conventional inorganic photoconductors, and there is a lot of flexibility in functional designs because of various choices for photoconductor materials including charge transporting material, charge generating material and binding resin, etc.
There are two types of organic photoconductors: single-layer photoconductor in which charge transporting material (hole transporting material or electron transporting material) is dispersed with charge generating material in the same photosensitive layer and multilayer photoconductor in which charge generating layer containing charge generating material and charge transporting layer containing charge transporting material are laminated.
The single-layer photoconductor has been drawing attention recently because it includes charge generating material and charge transporting material in a single photosensitive layer thereby enabling to manufacture with a simple manufacturing process and also, optical properties can be improved and either one of positive charging and negative charging can be applied because of fewer layer interfaces.
In contrast, the multilayer photoconductor is mostly of negative charging type and the multilayer photoconductor of positive charing type has not been put to practical use. This is because the electron transporting material which excels in electron transporting function, is less toxic and highly compatible with binder resins has not been put to practical use. Although single-layer photoconductor in general has bipolar sensitivity of negative and positive, most of them are used for positive charging because electron transporting material has low transportability of electron.
In general, image forming is performed by charging a photoconductor (main charging step), forming a latent electrostatic image by exposing images (exposure step), developing the latent electrostatic image by using a toner (developing step), transferring the formed toner image to a transfer medium (transferring step) and fixing in the image forming apparatus using electrophotography. The residual toner on the photoconductor is removed by cleaning blade, and the like (cleaning step) and residual charge on the photoconductor is erased by charge removing lamp, and the like (charge removing step). The reversal development method in which a toner of the same polarity as of the charging polarity applied to the photoconductor is used in charging step to develop is widely used in digital image forming apparatuses.
When an electrophotographic photoconductor is used in a digital image forming apparatus of reversal development type, transfer voltage applied to the photoconductor in transferring step becomes opposite of the charging polarity of the photoconductor. Generally, the transfer voltage is not applied to the photoconductor directly but through a transfer medium (paper) and it is not applied until the transfer medium passes through the transferring step, however, controlling ON/OFF timing of transfer voltage is difficult and in some areas of anterior and rear ends of the transfer medium, transfer voltage is often applied directly. As stated another way, because the transfer voltage begins to be applied before leading end of the transfer medium comes to the position of the transfer unit and also, the transfer voltage is continually applied even when part of the transfer unit is exposed due to passing through of the rear end of the transfer medium, the transfer voltage is directly applied to the photoconductor in some areas of anterior and rear ends of the transfer medium.
For example, because polarity of the voltage applied in the transfer device is negative in the case of a single-layer photoconductor of positive charging, negative space charge remains in part of the photoconductor where negative voltage is applied. As described above, since the single-layer photoconductor has sensitivity in both polarity, negative space charge is erased in the next charge removing step.
However, when negative polarity sensitivity of the positive charging single-layer photoconductor is inappropriate (electron transporting material has low electron transporting function), negative space charge is not erased sufficiently, and lowering of electric potential is induced by the effect of space charge even the photoconductor is positively charged in the next charging step.
As a result of the lowering of electric potential, lowering of the electric potential after exposure occurs in the exposing step and image defects (image degraded by transfer) such that the image density of the corresponding area is increased after developing by the toner occur.
The single-layer photoconductor is positively charged uniformly in the next charging step after going through the exposing step and developing step and then the charge on the surface of the photoconductor is erased uniformly in the charge removing step in general. However, when the polarity sensitivity of opposite charge of the positive charging single-layer photoconductor is inappropriate, space charge density of opposite of the charge polarity in the area where image is exposed is increased more than in the area without exposure, lowering of electric potential occurs in the next charging step and image degraded by image exposure tends to occur. When the single-layer photoconductor is used in the image forming apparatus which does not include charge removing step, the density difference between exposed area and non-exposed area in the image degraded by image exposure becomes further notable because the photoconductor surface is not exposed uniformly.
Techniques in which types of the constituent material of the single-layer photoconductor and a range of optical sensitivity difference between positive and negative are predetermined have been disclosed in Japanese Patent (JP-B) Nos. 3532808 and 3638500, Japanese Patent Application Laid-Open (JP-A) Nos. 2001-255678 and 2001-312075, for example, to solve above described problems. However, in any of the techniques, electron transporting function of the electron transporting material is not sufficient and resulted image defects by transfer and image exposure are not sufficiently prevented.
Moreover, illustrations of preventing and reducing image defects by assigning predetermined composition and condition of the transfer unit for the image degraded by transfer have been proposed. (JP-A Nos. 11-24446, 2000-242089, 2001-215818 and 2002-49194 and JP-B No. 3538389, for example). However, transfer units of complicated mechanisms are required, and compactification or cost reduction may be difficult, transfer efficiency of the toner may be deficient, and dust may be generated in transferring of the toner, thereby being insufficient as fundamental techniques for preventing image degraded by transfer.
As described above, satisfactory result for the technique for preventing or reducing image defects caused by transfer and exposure of the electrophotographic photoconductor having bipolar optical sensitivity of positive and negative has not been obtained in the present circumstances.
In recent years, spherical toner of the small diameter is attracting attention with the demand for higher image quality of the market. However, because properties of such spherical toner of the small diameter notably changes on the photoconductor and tend to cause cleaning defects, and the like, it is a contributing factor for image degradation due to toner filming or fusion and also is an important problem.
The methods in which fluorine resin particles are contained in a surface layer of the photoconductor as a lubricant agent to produce surface releasing effect for reducing coefficient of surface friction such as the ones proposed in JP-A Nos. 5-45920 and 2000-19918, for example, are very effective for solving the above problem of the negative charging electrophotographic photoconductor.
And because the same problem also occurs in the positive charging electrophotographic photoconductor, a photoconductor which has high tolerance to image degradation due to toner filming or fusion without causing cleaning defects even when a spherical toner of small diameter is used in the positive charging electrophotographic photoconductor is needed.
As an electron transporting material in the organic photoconductor, a new naphthalene carboxylic acid derivative expressed by the following Structural Formula (A) and an electrophotographic photoconductor of high sensitivity using the above material has been proposed (JP-A No. 2005-154409).

Where, in the above Structural Formula (A), X and Z represent groups each selected independently from hydrogen atom, substituted or unsubstituted alkyl group, substituted or unsubstituted cycloalkyl group, substituted or unsubstituted aralkyl group and substituted or unsubstituted aryl group and Y represents substituted or unsubstituted alkylene group or substituted or unsubstituted cyclo alkylene group.
Although the electrophotographic photoconductor in this example has higher sensitivity than that of the conventional photoconductor, there is no description about sensitivity and only durability of approximately 5,000 pieces has been evaluated.
An image forming apparatus equipped with a magnetic brush roller by which a photosensitive layer having a charge injecting layer is disposed on a conductive base substrate through which a light is transmitted so as to be facing the photoconductor to form a developing nip with the photoconductor, and charging and developing are performed by the developing nip and an exposing unit which exposes the photosensitive layer from the back side has been proposed in which a direct voltage superimposed with alternating voltage is applied to the magnetic brush roller to pull back the toner in a non-image area on the photoconductor drum to the developing sleeve (JP-A No. 6-202412).
In this image forming apparatus, however, direct voltage of −200V and alternating voltage of sine wave of 1,800 Hz frequency and 2 kV voltage between peaks are needed to be superimposed in order to perform charge injection because it has OPC photoconductor of negative charging type and positive charging cannot be performed.
A charging device which has a magnetic brush charger equipped with a magnetic brush of magnetic particles which comes in contact with charged body having a charge injecting layer of 109 Ω·cm to 1014 Ω·cm on its surface to charge the charged body, a voltage applying unit which applies a voltage to the magnetic brush charger and a resistive element which is connected in a charging circuit which has the charged body, has been proposed. In this charging device, the separation of conductive magnetic particles from the magnetic brush is prevented or reduced in order to prevent degradation of charging ability due to separation of conductive magnetic particles by having a resistance of the magnetic particles of 106 Ω·cm to 109 Ω·cm, the resistive element of 0.5 times or more of the resistance of the magnetic brush charger alone and the resistance of the charging circuit of 107 Ω·cm or less and by having appropriate environmental stability such that the resistance variation of the resistive element is smaller than the resistance variation of the magnetic brush charger alone relative to the variation in hygrothermal environment of 15° C. and 10% RH to 33° C. and 90% RH (JP-B No. 3495839).
However, charge injection to the OPC photoconductor to which a charge injecting layer is disposed on its surface is only exemplified by applying a direct voltage of −700V, and there is no example of positive charging for this charging device.
An image forming apparatus, which has an electrophotographic photoconductor, contact charging member, exposing unit, developing unit and transfer unit, in which a surface layer of the electrophotographic photoconductor contains a resin which is formed by polymerization of curable acrylic monomer or oligomer having a reactive acrylic group or methacrylic group and a conductive fine particle, a surface of the conductive fine particle is processed with a coupling agent having a reactive acrylic group or methacrylic group and appropriate charge injection can be performed by charge injection, has been proposed and excellent images can be obtained and duration of life of the photoconductor is sufficiently prolonged by the image forming apparatus (JP-A No. 11-95474).
However, charge injection of −680V is only exemplified by applying a DC voltage of −700V from the charge bias power source, and there is no example of positive charging for this image forming apparatus.
A color image forming apparatus in which charge injection is performed by applying a low voltage through a magnetic brush which is in contact with a photoconductor drum having a charge injecting layer on an outer side of the photosensitive layer has been proposed. In this color image forming apparatus, the residual toner which remains after transferring a toner image on the photoconductor drum is collected by developing devices corresponding to toners of each color for toner recycling (JP-A No. 2001-125375).
However, in this color image forming apparatus, image blur tends to occur during repetitive use in high temperature and high humidity environment because of the surface protective layer having diamond carbon structure containing hydrogen or non-crystalline carbon structure and in addition, ozone tends to occur because it is charged at −500V by charge injection.
A charge transporting agent for electrophotographic photoconductor containing a diphenoquinone compound expressed by the following Structural Formula (B) has been proposed.

Where, in the above Structural Formula (B), R1 to R8 represent one or two or more types selected from hydrogen atom, alkyl group, cycloalkyl group, aryl group, amino group and alkoxy group having 1 to 20 carbon atoms, however, all R1 to R8 are not to represent hydrogen atoms simultaneously.
The charge transporting agent for electrophotographic photoconductor has a problem of slow sensitivity when a sensitivity of the multilayer electrophotographic photoconductor is measured by positive charging.
In general, “electrophotographic process” is one of image forming methods in which a photoconductive photoconductor is charged with a corona discharge in dark place, for example and an image is exposed, a latent electrostatic image is obtained by selectively dissipating the charge of exposed area only and the latent image is developed by a detective particle (toner) consisting of colorants such as dyes and pigments and binding agents such as high-molecular substances to visualize and form an image.
In recent years, higher durability is demanded as well as higher image quality for the image forming apparatus using electrophotographic process such as electrophotographic copiers and electrophotographic printers.
Life duration of the image forming apparatus using the electrophotographic process as described above is often determined by the photoconductor.
Since the charging ability of the photoconductor is gradually degraded or electric potential of the exposed area is increased due to repetitive cycle of charging and exposing for extended period, sufficient electrostatic contrast for forming a latent electrostatic image cannot be obtained.
In order to solve this problem, it is necessary to improve the charge transporting material, a main material for forming a photosensitive layer. Even though initial properties are appropriate, when electrostatic fatigue of the photoconductor is added in the repeatedly performed electrophotographic process, sufficient properties cannot be obtained with the photoconductor in which conventional charge transporting material is used.
Another contributing factor for determining durability of the photoconductor includes wear of the photoconductor with time. This is because the photoconductor is gradually degraded and becomes worn by receiving mechanical and chemical influences in repetitive steps of charging, exposure, developing, transferring and cleaning in the electrophotographic process.
As wear of the photoconductor is progressed, it leads to lowering of charging ability and causes degraded images. Therefore, it becomes extremely important to use a photoconductor which excels in wear resistance in order to achieve higher durability of the photoconductor in the image forming apparatus using the electrophotographic process.
Many techniques in which protective layers are disposed in order to improve wear resistance of the photoconductor have been proposed (JP-A Nos. 1-205171, 7-333881, 8-15887, 8-123053 and 8-146641).
Even though wear resistance is improved by these proposals, electric potential of the exposed area is increased due to repetitive use of the photoconductor for prolonged period and image degradation such as lowering of image density occur. Furthermore, although wear resistance of the protective layer is improved due to high mechanical strength, when a foreign material is attached on a photoconductor surface for some reason, scratch tends to occur, causing image defects and it may be difficult to be used in the electrophotographic process.
The toner of small particle diameter is being used increasingly for higher image quality in recent years. This is because the image quality is remarkably improved by using the toner of small particle diameter. By contrast, removal of toner on the photoconductor is difficult due to small particle diameter of the toner and in addition, unremoved toner on the photoconductor tends to be fixed on the photoconductor.
Moreover, accumulation of the paper dust from the used paper and attachment of the foreign material such as agglomerated product of toner additives or other foreign materials on the photoconductor have been observed sometimes with higher durability of the information forming apparatus.
When these phenomenon occur, output image is degraded and so-called image defects occur. If such image defects occur, it is considered that the operating life of the image forming apparatus is over on the spot even though the photoconductor may have high durability.
With that, a method in which removal of foreign materials such as residual toner on the photoconductor after transferring or paper dust, etc. is emphasized to improve cleaning condition of the photoconductor. However, if the cleaning condition is improved more than is necessary, cleaning defects may occur at an early date, often causing image defects adversely because of abnormal wear or more roughened surface of the photoconductor.
For example, when toner removal capability is improved by conventionally used cutpile brush, a firm thick brush with a thickness of original yarn must be employed. However, if a thick yarn is used to form a cutpile brush, an edge of the fracture cross section comes in point-contact with the photoconductor, the surface of the photoconductor is scratched and abnormal wear occurs leading to image defects.
Moreover, when the cleaning condition is improved more than is necessary, scratches caused by repetitive friction is increased and problems of various image defects caused by the scratches occur such as streak defects caused by residual toner after transferring which slips through the scratched part in the cleaning unit or microscopic spotty defects caused by toner particles which get into the scratched part and become fixed, for example.
These image defects tend to increase when the toner of small particle diameter which is being increasingly employed for the purpose of achieving higher image quality, particularly when spherical toner such as polymerization toner is used and it had been extremely difficult for simultaneous pursuit of images of high quality without occurrence of image defects, and high durability for maintaining images of high quality.
In order to obtain an image forming apparatus of high image quality and durability, a cleaning device which produces no scratches on the photoconductor surface by making full use of the durability as well as changing the main material of the photoconductor, which is a cornerstone of image forming, to improve electrostatic durability for higher durability are indispensable. However, the image forming apparatus of high image quality and durability has not been obtained because it was impossible to fulfill these requirements.
The toner of small particle diameter and a small particle diameter distribution is known to be used for achieving higher image quality. However, the conventional pulverization toner is manufactured by melting/mixing colorants, charge controlling agents and offset preventing agents, etc. in a thermoplastic resin to be dispersed uniformly and by pulverizing and classifying the obtained composition. There are problems in this type of manufacturing method of the toner such that the particle diameter distribution tends to cover a wide range and fine and coarse powders must be removed by classification for obtaining duplicated images with an appropriate resolution and tone, resulting in very low yield. To solve the above problem, measures as disclosed in JP-A No. 9-222750, for example, have been taken, but they are not necessarily sufficient.
An organic photoconductor (OPC) containing charge generating materials and charge transporting materials have broadly been in practical use as an image bearing member used for the image forming apparatus for reasons such as no pollution and low cost. Hole transporting material and electron transporting material, etc. have been known as the charge transporting materials, but when the hole transporting material is used, negative charging process becomes indispensable and the photoconductor is degraded by ozone generation, etc. When the electron transporting material is used, a problem arises such that the carrier mobility is lower than that of the hole transporting material. In order to solve the problem of the charge transporting material, the use of predetermined charge transporting material has been proposed in JP-A Nos. 1-206349 and 5-142812, however, a problem arises in terms of compatibility with the binder resin.
In the image forming apparatus in which images are formed by using indirect electrophotography as exemplified by facsimile, laser beam printer, copier, and the like, each unit of charging, image exposure, development, transfer, separation, cleaning and charge removal is arranged centering around an electrophotographic photoconductor (hereinafter, may be referred to as “photoconductor”) and images are formed by sequential operations of each unit toward the photoconductor.
There is growing demand for higher image quality and durability of the image forming apparatus recently. In order to achieve compactification and higher durability, improvement of the charge transporting material, a main constituent material in the photosensitive layer of the photoconductor is indispensable. This is because the charging ability of the photoconductor is gradually degraded or electric potential during exposure is increased due to repetitive loads added by each unit such as charging and exposing, etc. and sufficient electrostatic contrast for forming a latent electrostatic image cannot be obtained.
At the same time, it is necessary to improve mechanical durability against repetitive operations as well as to improve repetition property of the photoconductor relative to static electricity in order to improve durability of the image forming apparatus as a whole. This is because a slight twist or warp may appear on the photoconductor by the force added for rotating the photoconductor for extended period.
The drive power is transmitted to the photoconductor through a member which is normally set at both ends of the photoconductor, a flange, and slight twist or warp appears at the flange joint after performing drive power transmission for extended period and runout accuracy during rotation of the photoconductor is changed. This slight twist or warp has been observed in the conventional image forming apparatus though it does not cause any problems, in some cases however, this slight twist or warp affects output images of the image forming apparatus which is demanded recently, particularly the full-color image forming apparatus which uses a high-level technique of color lamination.
Moreover, in order to suppress twist or warp of flange joint as described above, the size of the flange member for strengthening the joint may be slightly made equal to or more than the inner diameter of the drum, which is a support of the photoconductor. When this method is employed, however, problems such that the circularity of the drum is changed or runout accuracy is degraded arise due to force added when the flange is mounted. Furthermore, fixation of the flange by using an adhesive bond which has been operated conventionally also causes a slight change in accuracy of the flange joint due to contraction or expansion generated during fixation of the adhesive bond.
Other than this, resonance generated during charging or frictional noise caused by contact with the cleaning member have also been a problem.
The charging of the photoconductor will be explained below. The photoconductor is charged (electron is provided) at −300V to −800V by means of a charging unit. There are two methods for applying a voltage to the charging unit: a method in which direct voltage is applied and a method in which direct voltage superimposed with alternating voltage is applied. Although forming an image which is practically usable is possible by applying direct voltage alone, the image can be more independent of the environmental condition and uneven electric potential, which is thought to be caused by irregularity of the charging member and the photoconductor and slight nonuniformity in the member which are produced when contact charging is employed, can be considerably reduced.
The charging method generally employed now include corona charging in which a photoconductor is charged by applying a high voltage of approximately −4,000V to −6,000V to a metal wire with a diameter of 40 μm to 80 μm which is extended in a shield case such as tungsten wire and nickel wire, contact charging in which a photoconductor is charged by applying a direct voltage of −1,200V to −2,000V or a direct voltage of −500V to −900V superimposed with an alternating voltage of 1,000V to 2,500V/500 Hz to 4,500 Hz to a charging member having a resistance of approximately 102 Ω·cm to 108 Ω·cm which is roller-shaped or brush-shaped, and a proximity noncontact charging in which a photoconductor and a charging member are arranged so as to have a distance of approximately 30 μm to 250 μm in between and the photoconductor is charged by applying the above voltage.
In corona charging, high density ozone is generated because of the application of high voltage, causing environmental problems by ozone odor or contamination problem of metal wires such as the tungsten wire and nickel wire due to discharge product produced by repetitive use. Because of this, the contact charging in which a photoconductor can be charged by applying a low voltage is employed recently and ozone generation is reduced to as low as 0.1 ppm or less. Therefore, many image forming apparatuses use contact charging which produces less ozone in recent years and direct voltage superimposed with alternating voltage is applied to the charging member.
However, when direct voltage superimposed with alternating voltage is applied to the charging member, noise problem caused by annoying charging noise generated during charging arises other than ozone and nitrogen oxide which cause image quality degradation. This charging noise is not generated with direct voltage, and it is a unique phenomenon related to oscillating current and as the amplitude increases and the material of the support in the photoconductor transmits more sound, charging noise increases. Therefore, it is desirable to set the condition so as to lower the noise as much as possible, however, the condition will be toughened as charging stability is increased, resulting in an increase of charging noise, and it is indispensable to take measures to solve above noise problem.
Methods such as the one in which the support in the photoconductor is thickened, vibration suppressing material (filling material) is inserted inside of the drum-shaped photoconductor, or charging member side is improved, etc. have been proposed as measures to improve the above phenomenon. These methods in which generation of charging noise (high-frequency sound) during charging is improved by inserting an vibration suppressing material inside of the drum-shaped photoconductor for suppressing sound transmittance in the photoconductor and shifting resonant frequency toward the range where it is not well transmitted to human ears have been proposed as follow. Examples include (1) a method in which cushioning material is pressed and inserted inside of the photoconductor drum (JP-A No. 63-60481), (2) a method in which viscoelastic material is filled inside of the photoconductor (JP-A No. 3-105348), (3) a method in which a rigid body of 2.0 g/cm3 or more density is inserted inside of the photoconductor (JP-A No. 5-197321), (4) a method in which a member consisting of two or more elastic bodies (O ring) and cylindrical member [a plastic with 1.5 or more specific gravity (polybutylene terephthalate resin containing 20% or more of glass fiber)] is inserted in the photoconductor (JP-A No. 11-184308) and (5) a method in which a resin-made cylindrical member having a built-in metal spring is inserted and fixed to the inner wall of the photoconductor by suppress strength (JP-A No. 2000-321929).
A method in which a support in the photoconductor has an vibration suppressing member and the thickness other than that of vibration suppressing member is set at 1.9 mm or more has been proposed as a method for enhancing vibration suppressing effect by thickening the thickness of the support in the photoconductor (JP-A No. 2000-19761). Moreover, a method in which deposit density of the photoconductor is set at 0.6 g/cm3 or more and 2.0 g/cm3 or less to obtain vibration suppressing effect has been proposed (JP-A No. 2000-155500). Moreover, a method in which the charging noise is improved by disposing a coating layer on a surface of the hollow charging member (roller) and inserting elastic body inside of the charging member to have a structure in which a cored bar is supported through the elastic body has been proposed as a method for achieving charging noise suppression through charging members (JP-A No. 9-230671). Furthermore, measures will be different depending on whether the charging noise is suppressed by shifting the vibrational frequency toward the area where it is not annoying to human ears or vibration itself is suppressed.
The above methods have effects to a greater or lesser degree respectively. Though the charging noise is improved in the contact charging, it may not be as effective in noncontact charging in which the charging member is arranged to be close to the photoconductor. For example, sufficient effect are not likely to be obtained by a method in which charging noise is suppressed by the charging member or just by thickening the support in the photoconductor. The method in which the sound (noise) is suppressed by inserting a filling material inside of the support in the photoconductor may be effective, however, when a space still exists between the support and the inserted material after insertion, expected effect is not likely to be obtained when the weight is small. Moreover, the effect may be lowered when the constituent member is of single composition. Furthermore, environmental problems mush be taken into account in recent years and recycling and reuse are required and should be in consideration.
Moreover, the support is slightly deformed when the above filling material is inserted inside of the support for suppressing vibration. This is because the support is pressed by the filling material and deformed slightly when there is a need to attach the support and the filling material firmly to eliminate the space which exists between the support and the filling material for suppressing noise.
As described above, annoying noise caused by sympathetic vibration and sympathetic resonance may be heard during charging depending on charging method. In addition, frictional noise or fluttering noise generated by a member which comes in contact with the photoconductor such as cleaning blade has also been heard during rotating of the photoconductor. A method in which sound (noise) is suppressed by inserting a filling material inside of the photoconductor and increasing the mass as described above has been employed as a measure to this kind of the problem, however, slight degradation of the photoconductor accuracy is unavoidable as described above and therefore, it is not satisfactory for fulfilling the increasing demand of higher image quality in recent years.
There are two types of the full-color image forming apparatus using electrophotography known in general. One is called single type, or single drum type and the apparatus is equipped with one electrophotographic photoconductor (hereinafter, may be referred to as “photoconductor”, “electrophotographic photoconductor”, “image bearing member” or “latent electrostatic image bearing member”) and 4 developing members which correspond to 4 colors of cyan, magenta, yellow and black. In the single method, toner images of 4 colors are formed on the photoconductor or recording medium. It is possible to standardize charging member, exposing member, transfer member, cleaning member and fixing member arranged around the photoconductor, allowing more compact design at low cost as compared to the tandem type mentioned below.
The other type is called tandem type or tandem drum type (JP-A No. 5-341617). The apparatus of this type is equipped with multiple photoconductors. In general, each one of charging member, exposing member, developing member, transfer member and cleaning member are arranged for one photoconductor drum to form one image forming element and multiple image forming elements (four in general) are mounted in the apparatus. In the apparatus of tandem type, a toner image of single color is formed by one image forming element and toner images are transferred to a recording medium sequentially to form a full color image. The tandem type has an advantage in being able to form an image at high speed. This is because toner images of each color can be formed in parallel. Therefore, image forming process only takes about one-fourth of the single type, and it is applicable for four times as much of high speed printing. Moreover, it also has an advantage of substantively improving durability of each member contained in the image forming elements such as photoconductor. In single type, each step of charging, exposure and developing is performed 4 times by using one photoconductor to form a full color image and in contrast, the above operation is performed only once by using one photoconductor in tandem type.
However, the tandem type also has disadvantage in terms of size of the apparatus which grows in size because image forming elements are arranged in plural numbers, thereby making it costly.
In order to solve the above problem, one image forming element has been minimized in size by downsizing the photoconductor and each member arranged around the photoconductor. By doing this, material cost has also been reduced as well as downsizing of the apparatus and overall cost of the apparatus has also been reduced in some degree. However, a new problem has arisen such that the photoconductor included in the image forming element must be of high sensitivity and stability must also be increased considerably with compactification and downsizing of the above apparatus.
It is necessary to perform an image forming process at high speed for achieving higher speed printing, which is the first purpose of the tandem type. Therefore, it is necessary to be optically attenuated rapidly when being exposed because of sensitivity and charged condition of the used photoconductor. Furthermore, it is necessary for the optical attenuation property to be maintained stably during repetitive use. Particularly in the full color electrophotographic apparatus of tandem type in which image forming elements are arranged in plural numbers, a particular color (black, for example) is generally used in large amount according to the percentage of colors printed by users and burden on each photoconductor are not uniform. In this case, optical attenuation property of some of the photoconductors is degraded resulting in tone change when a full color image on which colors are overlapped is printed during repetitive use.
As described above, quality of printed image becomes inappropriate when the property of the photoconductors during repetitive use is unstable, causing degraded images such as tone change and background smear during repetitive use for prolonged period.